Method and apparatus for electric ignition



July 7, 1 5. J. SHEHE EN METHOD AND APPARATUS FOR ELECTRIC IGNITIONFiled Dec. 6, 1955 INVENTOR. SHAY J. SHEHEEN ATTORNEY U i d t s Patc l QMETHOD APPARATUS FOR ELECTRIC IGNITION Shay J. Sheheen, Ithaca, N.Y.,assignor to General Laboratories Associates, 'Inc., Norwich, N.Y., acorporation of New York i Application December 6, 1955, Serial No.551,380 A 3 Claims. (Cl. 313-431) The present invention relates to amethod and apparatus for electric ignition, and is particularlyconcerned with a method and apparatus for producing an electric spark ina fluid medium, e.g., a fuel or a fuel mixture, 'whichmedium may beeither electrically conductive or non-conductive.

In certain types of modern engines, for example, rocket engines, thefuel may contain one or more electrically conductive components, forexample, nitric acid. Further-' more, it may be that the component inquestion is conductive inone state, e.g., the liquid state, andnon-conductive or discontinuous in another state, e.g., the vapor state,and both states may be present in the fuel to be ignited; Suchelectrically conductive components efiectively' shunt the electrodes ofconventional spark plugs, thereby preventing proper ignition of the fuelby the plug.

Conventional fixed gap spark plugs in current use may be classified aseither jump gap plugs or surface gap plugs.

A jump gap plug comprises two sparking points separated by an air gap.While the gap is nominally termed an air.

gap, under operating conditions it is filled by an electricallynon-conductive mixture of air and fuelgas or vapor,

which mixture is ignited by a spark between the points of the plug. Ajump gap plug must be supplied with electrical energy from a sourcehaving a potential high enough to break down the insulating materialbetween the points of the gap. That potential is commonly termed thebreakdown voltage of the gap, and is much higher than the potentialrequired to maintain the spark after the gap has broken down; p

A surface gap plug comprises two electrodes separated by a member havinga layer of surface conductive material. Such surface conductivematerials are usually materials of the class generally known assemi-conductors. When a surface gapis subjected to asufiiciently highvoltage, an are forms across the surface of the material, producing asparkwhich is communicated to the fuel.

(Spark plugs with gaps of suchsurfac'e conductive mate rial are incommon use in jet engines.) The potential of the source supplying such aplug is commonlymuch lower, for a given gap width, thanthe breakdownvoltage apparatus described herein by providing a spark plughaving asurface conductive gap and a jump gap in parallel with it. The plug isprovidedwith electrical energy from a low impedance source typical ofthose commonly used in connection with surface conductive plugs. thepotential of the source is lower than the 2,894,161 PatntedJuly'l, 1959I breakdown voltage of the jump gap and the current capacity of thesource is substantially higher than that commonly used with jump gaps. i3

Such a plug in combination with such a source will produce a sparkwhether the fluid medium to which the plug is exposed is electricallyconductive and completely shunts the plug or whether that fluid mediumis nonconductive, for example, by reason of being in a discontinuousstate such as a vapor. If the fluid medium is nonconductive, the sparkappears only at the. surface gap and i if the fluid medium is conductivethe spark appears only at thejump gap. 1,; p j

Other objects and advantagesof the. invention will become apparent froma consideration of the following description and claims, taken together,with the accompanying drawing.

In the drawing:

Fig. l is an electrical wiringdiagram of acircuit for energizing a sparkplug in accordance with the invention; and i i i Fig. 2 is a centralcross-sectional view througha spark plug embodying the invention.

The circuit illustrated inFig. l is a simplified version of the ignitionsystem shown and claimed in the patent to John V. McNulty, No.2,716,720, dated August 30, 1955,

. entitled Engine Ignition Apparatus and Procedure. Sys-" tems of thistype have the characteristic that charges of electricity for theformation'of sparks are delivered to the spark plngsin carefullymeasured and regulated quantities of energy,.so that uniform sparks maybe produced at the plug, regardless of variations which may occur in theener gy supply system, for example, in the potential of. the source ofelectrical energy; i

Referring to the drawing, there'is shown a battery 1- which supplieselectrical.enengytoia motor 2, drivingly connected to a multiple lobecam 3 and a single lobe cam 4.. i The multiple lobe cam 3 operates aswitch 5 connected in series between the. battery 1 and the primarysection 6 of the winding of an auto-transformer 7. The

secondary section 8 of the winding of the autmtransformer 7 is connectedin series with a circuit which includes a,

rectifier or other asymmetric element 9, and a condenser 10. An outputcircuit is connected across the condenser 10, and includes a triggerswitch 11 operated by the single lobe cam 4, and a spark plug 18including a jump gap 19 anda' parallel surface gap 20. i i i The motor 2has a voltage-speed characteristic such that the speed varies directly,and linearly .as the voltage.

Consequently, the time ofclosure of the switch Sbythe lobe of the cam 3varies inversely with the voltage of the source 1. Regardless ofvariations in the voltage of the source, each closure of the switch 5therefore delivers a fixed predetermined quantity of electricity to theprimary section 6 of the winding of the auto-trasformer 7. Thesemeasured quantities are stepped up in potential by the auto-transformer,and pass through the rectifier 9 andare stored on the condenser 10. Thecondenser 10 is discharged each time that the single lobe cam 4 actuatesthe trigger switch 11. Sincethe single lobe cam 4 is driven by the samemotor 2 which drives the multiple lobe cam 3, it will be apparent thateach discharge of the condenser 10 is preceded by the same number ofcharging pulses, determined by the number of lobes on the periphery ofthe cam 3.

Referring to Fig. 2, there is shown in detail the spark plug 18including an outer cylindrical'casing 12, an annular ceramic body 13having a coating 14 of semiconductive material on one face and an innerthreaded post 15 concentric with the casing 10.

'Attached to the outer casing 12 near the end thereof" is a rod 16 ofelectrically conductive material having its outer end bent over towardthe axis of thec asing 12,

as shown at 164. "Another rod 17 is welded or otherwise suitablyfastened to a flange 15a carried at the end of the post 15 and projectsinto close proximity with the tip of rod 16a. The tips of the rods 16and 17 define between them" the jump gap 19. a

The post- 15 'supports a conductive washer 20a which is held inconductive contact with the face of the semiconductive coating 14 by theflange 15a of the post, the shank of the post 15 being threaded into themain inner conductive member 15b the end of which bears on the oppositeface of the ceramic member 13. An annular ring 20b disposed between ashoulder of the casing 12 and the face of the semi-conductive coating 14also is held in conductive contact with this coating. The outer diameterof the washer 20a and the inner diameter of the ring 20b are such thatthe annular space therebetween determines the surface gap 20. Thisspacing and the spacing of the flange 15a from the casing 12 are greaterthan the jump gap 19.

When used on a rocket engine, the plug described above may at times becompletely immersed in electrically conductive liquid fuel. At othertimes, it may be immersed in a spray comprising particles of such fuelseparated by regions of vapor. At still other times, it may be immersedin vapor. In either of the latter two cases, the vapor path between theelectrodes of the plug may be continuous or discontinuous. At any ofthese several times, a spark may be required at the plug, for thepurpose of igniting the fuel.

The gap 19 is constructed so that its breakdown potential in air orother non-conductive gaseous fluids is substantially greater than thepotential to which the condenser is charged. Consequently, when thespark plug 18 receives a pulse of electrical energy at a time when theplug is immersed in a non-conductive fluid, there is no spark generatedat the jump gap 19, but a spark is generated at the surface gap 20, inthe conventional manner for surface gap spark plugs.

When the plug is immersed in a conductive liquid, surface gap 20'iseffectively shunted by that liquid, so that the current flow through thesemi-conductive layer 14 is too small to initiate a spark. The lowestimpedance path between the outer electrode or shell 12 of the plug andthe inner electrode or post is then the path through the rods 16, -17and the conductive liquid at the jump/gap 19. The current thereforetends to concent rate in that path. The energy discharge from thecondenser 10 is substantially greater than that normally used inconnection with gap plugs and issufiicient when passing through theliquid in the jump gap 19 to generate gas .or vapor'in'the gap.

The particular mechanism by which this gas or vapor is produced is notknown with certainty. However, most of the liquids on which tests havebeen made have been electrolytically conductive, and it is consideredmost probable that the gas or vapor is generated by electrolysis of theliquid, with the rods 16, 17 acting as anode and cathode. It ispossible, however, that with some liquids, the gas or vapor may beformed by some other physical phenomenon-40m example, a portion of theliquid may become heated above its'boiling point. This vapor appearsfirst only in a very small volume within the gap, so that the initialvolume of vapor has a very low breakdown potential and a spark readilystarts in the vapor as soon as it is formed. The spark generates furtherheat and the volume of vapor rapidly expands between the points of thegap, until the spark completely fills the jump gap. A spark ofsubstantial volume and'energy is obtained even under liquids which arehighly conductive.

A jump gap 19 should be small in area and short in length as compared tothe surface conductive gap. The maximum length of jump gap which maybe'used is a function of the gap conductivity and the energy of thesource; a

The length of the surface conductive gap, i.'e., the

radial distance between the shell 12 and the post 15, also depends onthe energy of the source- Experiments have been successfully conductedusing a plug and an energy source of the type described with a potentialof approximately 1000' volts and an energy per spark discharge of 2joules, with a jump gap 0.030 in. long and a surface gap 0.070 in. long.

Whi e I ha Sh wn an described a Pre rr d o ment of my invention, othermodifications thereof will electrodes are immersed in said electricallynon-conduo tive fluid medium, and a pair of points remote from saidmember, respectively electrically connected to said electrodes andspaced apart by a distance substantially smaller than the spacingbetween said electrodes and constituting a jump gap, said jump gaphaving a breakdown potential in said non-conductive medium substantiallygreater than that of said discharge path, said jump gap being effectivewhen said points are immersed in said electrically conductive fluidmedium to provide a second electrical energy discharge path between saidelectrodes of lower impedance than said first path, said jump-gap havinga limited cross-sectional area and being elfective upon a discharge ofcurrent therethrough to change a portion of said conductive mediumadjacent said gap to a gaseous state, and to create a spark dischargewithin said changed portion of the medium.

2. A spark plug for producing a spark in either electricallynon-conductive or conductive fluid media, comprising a cylindrical shellof electrically conductive material, an annular body having at least oneend of semiconductive material within and contacting said shell andconcentric therewith, with said one end disposed toward and spacedinwardly from an end of the shell, a central post of electricallyconductive material extending axially of and contacting saidsemi-conductive material and having an end projecting beyond said body,means .to connect said shell and said post to the opposite terminals ofa source of "electrical energy having a potential sufficient when theplug is immersed in said non-conductive medium to produce a sparkbetween said end portion of said post and said shell and across thesurface of said one end of Y said body, a first electrically conductiverod attached to said post and extending axially therefrom beyond the endof said shell, a second electrically conductive rod attached to saidshell and extending beyond the end thereof and parallel to said firstrod, at least one of said rods being bent to bring the rod ends towardone another to define between said rod ends a jump gap having abreakdown potential in said non-conductive medium greater than thepotential of said source, and a breakdown potential in said conductivemedium at least as low as the potential of said source.

3. A spark plug for producing a spark in either electricallynon-conductive or conductive fluid media, comprising a cylindrical shellof electrically conductive material, an annular body having at least oneend of semiconductive material within and contacting said shell andconcentric therewith, with said one end disposed toward and inwardlyfrom an end of the shell, a central post of electrically conductivematerial extending axially of and contacting said semi-conductivematerial and having an end portion projecting beyond said body, saidpost, shell and semi-conductive material defining a first electricalenergy discharge path having a predetermined breakdown potential whenimmersed in an electrically non-conductive fluid medium, said dischargepath being recessed inwardly from the end of said shell, a firstelectrically conductive rod attached to said post and extending axiallytherefrom beyond the end of said shell, a second electrically conductiverod attached to said shell and extending beyond the end thereof andparallel to said first rod, at least one of said rods being bent tobring the rod ends toward one another to define between said :rod ends ajump gap having a breakdown potential in said non-conductive mediumgreater than the breakdown potential of said first path, said gap havinga spacing narrower than the: spacing between said post and she'lladjacent said body.

References Cited in the file of this patent UNITED STATES PATENTS SmitsOct. 24, Togn'ola Oct. 16, Sackett et a1. Apr. 8, Devaux Sept. 1, LairdFeb. 7, Tognola May 22,

FOREIGN PATENTS Great Britain Nov. 1,

